This invention relates generally to gas turbine engines. More particularly, the present invention relates to thrust augmentation systems for mixed flow turbofan engines.
Turbofan engines utilize exhaust gases generated by a main turbine engine to rotate an additional turbine for propelling a drive fan. Thus, turbofan engines produce two flows of air used to create thrust: a primary airflow resulting from combustion in the main turbine engine and a secondary airflow accelerated by the drive fan. In mixed flow turbofan engines the primary airflow (hot air) and the secondary airflow (bypass air) are united in an exhaust duct to be simultaneously passed through an exhaust nozzle. The exhaust nozzle further accelerates the mixed flow exhaust.
Additional thrust can be generated in turbofan engines through the use of an augmentation system, also known as an afterburner or re-heater. Augmentation systems further accelerate the mixed flow exhaust utilizing the remaining oxygen in the exhausted air and an injected fuel stream to perform a secondary combustion process. Mixed flow augmentation can be performed in either the primary air or the secondary air streams. Typically, augmenters are used during short periods when auxiliary thrust is necessary, such as during take-off or when performing combat maneuvers such as intercept missions.
In general, augmented turbofan engines must increase the nozzle throat area for augmented operating conditions to prevent fan or low compressor surge, a condition in which a pressure pulse propagates forward into the fan or low pressure compressor, causing instability due to airfoil or endwall stall. Multi-stage augmenters can also be used to modulate fuel flow during augmented operation to reduce de-stabilizing pressure pulses and improve synchronization of augmentor fuel injection and exhaust nozzle area increases. Both of these solutions, however, increase the cost and complexity of the turbine engine.
For some applications, non-augmented mixed flow turbofan engines having fixed exhaust nozzles are desirable due, in part, to their reduced cost and complexity. Thus, for mixed flow turbofan engines, it is the current state of the art to use either a variable exhaust nozzle with augmentation, or a fixed nozzle without augmentation. There is, therefore, a need for a fixed nozzle area augmentation system for mixed flow turbofan engines.